Method and means for eliminating capacitive coupling



H. A. WHEELER METHOD AND MEANS FOR ELIMINATING CAPACITIVE COUPLING Filed Feb. 2'7, 1925 2 Sheets-Sheet l INVENTOR Mia/4% /;Wm%%m ATTORNEYS May 6, 1930. H. A. WHEELER METHOD AND MEANS FOR ELIMINATING CAPACITIVE COUPLING Filed Feb. 2'7, 19 25 flu ' INVENTOR ATTORNEYS.

2 Sheets-Sheet 2 NM, UM

Patented ay i930 seas HAROLD A. WHEELER, OF WASHINGTON, DISTRICT OF COLUMBIA, ASSIGNOR TO vE.ZLZEL'IJINE CORPORATION, OF JERSEY CITY, NEW JERSEY, A CORPORATION OF DELAWABE METHOD AND MEANS FOR ELIMINATING CAPAGITIVEOOUPLING Applicationafiled February 27, 1925. Serial No. 11,973.

This invention is directed to the neutralizat on of capacitive coupling between electric clrcuits, especially where such coupling is unavoidableand causes undesirable reactions between the circuits.

I The invention is particularly applicable to audion circuits. The term audion as used herein is defined as any electric device comprising a cathode, herein referred to as the filament; an anode. referred to as the plate and an electrostatic control electrode, referred to as the grid. The arrangement of the electrodes is such that variations in the potential-of the grid relative to the filament affect the current between plate and filament in a manner equivalent to the proportional changes in the potential of the plate relative to the filament. Because of the finite dimensions of the audion, the capacity between any two of the electrodes is sufficient to be appreciable and is, in addition, usually increased by capacity between the external connections to said two electrodes. In a common case where one resonant circuit is associated with the grid and another resonant cir-' cuit is associated with the plate, this inherent capacity existingbetween grid and plate causes capacitii e couplingbetween thosetwo circuits,

which coupling may in turn cause undesirable reactions, such as so-called regenerative action or the generation of electrical oscillations which interfere with the operation of the system as a whole.

Capacitive coupling can, by means of' this invention, be neutralized and regenerative amplifiers in radio transmitting circuits, or

radio receiving sets. -Although several other methods havebeen suggested for this purpose, only a few of those methods satisfy the requirement that adjustments necessary for' neutralizing the capacitive coupling be inde..

pendent of the frequencies of alternating currents to which the system as a whole must be accommodated. The few other methods which do meet this requirement embody circuit arrangements essentially different from and more complex than those herein described, as will be clearly demonstrated hereinafter. This application is a continuation in part of application Serial No. 739,080, filed September 22, 192%.

It is the purpose of the present invention to neutralize capacitive coupling existing effectively between any two or more circuits by means of a rearrangement including the introduction of capacities, either inherent such as introduced by the juxtaposition of the wiring or othef component apparatus, such as audion electrodes, or in the form of added condensers where necessary, to form a network of pure capacities which can be adjusted so that there is'no resultant capacitive coupling between any two of said circuits. The principles and circuit arrangements utilized in the practice of this invention are hereinafter described in detail with the aid of illustrations of a preferred embodiment.

Fig. 1 is an elementary diagram of a pure capacity bridge illustrating the basic principle employed in this invention;

Fig. 2 is a circuit diagram showing one method of applying this invention to an audion circuit;

Fig. 3 illustrates an alternative method of applying this invention to an audion circuit;

Fig. 4 is a circuit diagram of a radio receiver employing a two-stage radio-frequency amplifier which utilizes the prlnclples illustrated in Fig. 3.

In Figs. 1 to 3, inclusive, reference characters A and 13 represent electric circuits of any nature whatsoever; i. e., tuned, untuned or resonant circuits. Likewise, reference characters C C 0 and C" represent capacities forming the four arms of a pure capacity bridge, and may be of any nature whatsoever, as, for example, condensers, or natural inherent capacities between circuit elements, or between audion electrodes. When one of these capacities is by preference solely a natural inherent capacity,it is shown connected in dotted lines such asC in Figs. 2 and 3.

- is satisfied, the balance will obtain regardless of whet-her the capacities be inherent, or

added in the form of condensers, or both combined. In the last case, it is the eflective value of the combined capacities which 1s to be considered in attaining a balance. For

example, capacity C of Fig. 2 is to be taken as including the inherent grid-filament capacity of the audion 1; and referring to Fig. 3, the bridge capacity G which is connected between the plate and the filament of the audion 3, must be taken as including the effective capacity existing between the plateand filament electrodes, and would thus comprise the inherent plate-filament capacity of the audion and the distributed capacity of the coil 4 and battery 12 added to the additional capacity or condenser symbolically represented at C The elementary diagram shown in Fig. 1 represents a pure capacity bridge comprising four capacities, C, C, C and C connected with two circuits A and B, these circuits comunderstood that others of the bridge capaciprising, respectively, the conjugate arms of the bridge. Reference to the figure will show that the circuit A is connected to one pair of conjugate points of the bridge and that the circuit B is connected to the other pair of conjugate points. The values of the capacities are adjusted .as necessary until no part of the voltagegenerated between the terminals of one circuit is impressed between the terminals of the other circuit. There must be no inductive coupling or resistance coupling between the two circuits if an accurate balance is to be obtained. The quantitative condition for an accurate balance or a neu v tralization of capacitive coupling between circuits A and B is:

inherent capacity or solely an added capacity, or both inherent and added capacities together. Inductive coupling, or resistance coupling. may then be introduced between the circuits if required, but the capactive coupling will remainneutralized as long as equation (1) is satisfied,

Fig. 2 shows the principle of this invention applied to a simple audion circuit Here, as in Fig. 3, reference characters A and B can be regarded as the input and output circuits, respectively, of an amphfier stage containlng an audion 1, which includes plate P, grid (i and filament F. For simplicity, the filament (F) is Shown 'in .this and the other figures to be of the simple hot-cathode type. It should be understood, however, that the present invention is equally applicable to an audion having an indirectly heated cathode, in which event the wordfilament. as used in the present specification and appended claims is, of course, intended to refer to such cathode.- Since, according to common practice, the input circuit of an audion is always connected to the grid and the output circuit to the plate, the natural capacity between the grid and plate ordinarily causes capacitive coupling between the audion input and. output circuits. By arranging the capacity bridge as shown, (which for convenience may be termed an input-circuit bridge) this capacitive coupling is balanced and neutralized, because the input and output circuits are, respectively," the conjugate arms of a balanced bridge. In this, as well as in the circuit arrangements illustrated in the other figures the capacity C may be considered as causing undesirable coupling to be neutral ized. The grid leak or high resistance 2 connected between the grid and the filament circuit is introduced for the usual purpose of maintaining a constant average grid potential with respect to the filament. Sucha resistance is a simple means for effecting the required grid potential, which may be either positive or negative with respect to the filament, as required by the particular audion employed, and may be obtained in many other ways, such as by the use of an impedance, or by a separate battery. Maximum amplification will be obtained when as high an input voltage as possible is impressed upon the grid of the audion, and it is therefore preferable in the arrangement of Fig. 2, that the ratio unity.

The circuit arrangement illustrated in Fig. 3 is in general similar to that shown in Fig. 2 except that the neutralizing bridge is. inverted with respect to the grid and plate,-and may be termed an output-circuit bridge. It may here be pointed out that the outputcircuit bridge for balancing and neutralizing undesired coupling between the input and output circuitseespecially the plate-grid capacity couplingof the common three-"electrode audion. will always include the natural inherent plate-filament capacity of the audion and will not include the grid-filament capacity thereof; while the input-circuit bridge above described in connection with Fig. 2 will include the natural inherent gridfilament capacity of the audion and will not include the plate-filament capacity thereof.

In this figure reference character A represents the input circuit connected to grid G and filament F of audion 3. Capacity C represents the natural inherent capacity existing between the grid G and plate P. Output circuit B. is associated with the capacities C C and C, which capacities are adjusted so that the balanced condition expressed by equation (1) between conjugate circuits A and B is obtained. A choke coil 4 of high inductance and low electrostatic capacity is inserted in the plate circuit between plate P and plate battery 12 in order to by-pass the direct plate current past the capacities of the bridge. but which at the same time offers a high impedance to radio frequency currents flowing in the plate-output circuit, thus keeping those currents from flowing through the 'lf battery 12. For convenience, the circuit P, at. 12. F may be called the plate circuit to distinguish from the alternating current output circuit B.

Fig. 4 shows a circuit diagram of a radio receiver employing two stages of tuned radiofrequency amplification and a detector, or rectifier. This-circuit arrangement utilizes the principle described above with reference to Fig. 3 and includes an output-circuit bridge. or capacity network in the plate-output circuit of each of the radio-frequency amplifving audions 16 and 17, respectively, of which the input circuit and output circuit of each audion form conjugate arms of a balanced bridge as previously described. The input circuit of the first audion 16 comprises an antenna and ground 29, a series condenser 28 and an auto-transformer T, of which the primary-to-secondary ratio may be about 1:2.8. This transformer is tuned by variable condenser connected across its extremities. The output circuit of this audion includes an inductance 19 tuned by a variable condenser. 20, and is coupled to the input circuit of the next suceeding audion 17 through a capacity 21. The second stage of amplification is similar to the first and is coupled to the input of the detector or rectifier audion 18 by capacity 22. In the plate circuit of each radio-frequency amplifier bethe grid and the filament circuit for rents across the fi tween the plate'electrode and plate battery If the It should here be noted that the usual purpose of maintaining a constant average gridpotential with respect to the filament. High-frequency by-pass condensers 23, 24 and 26 are provided to introduce a low resistance path to high-frequency curament battery 11, plate battery 12 and telephones 27, respectively. Maximum amplification in this arrangement will also be obtained when as high an input voltage as possible is impressed upon the grid .of the audion, but contrary to Fig. 2, it is here preferable that the ratio of capacities be small 0 As is well known, the output impedance of an audion is ordinarily much less than the input impedance thereof; furthermore, the maximum transfer of energy will be attained when the impedances, which are connected respectively in the output and input circuits of the audions, are chosen so as to be of values as nearlyas possible the same as the respective output and input impedance values of the audions. Accordingly, the capacity value of condenser C connected from plate to filament would usually be much larger than the capacity value of condenser C connected between the grid and filament of the next audion, but which would also usually be connected in series with a coupling device, such as condenser 21 or 22.

The capacity bridge represented in Fig. 4 as C C C and C, wherein C represents the undesirable grid-plate capacity to be neutralized, may, for example comprise capacities of the following values:

C 300 micro-microfarads 0 -f micro-microfarads;

C 3- micro-microfarads;

C" micro-microfarad 0 being an accidental inherent capacity of 3 micro-microfarads. .The other values of capacity mentioned, were found, when associated with C according to the present invention, to eliminate completely all tendency toward regeneration or feed-back due to that inherent capacity. Of course, values of capacity other than those just given might be used provided that the quantitative relation expressed in equation (1) be realized, and preferably that the ratios of capacities before mentioned bemaintained.

It should be noted that in the case shown in Figs. 2, 3 and l it is a distinct advantage to make the ratio ms small as possible with- 1 out materially affecting the associated input and'output circuits. For instance, in the amcompared with the potential drop across G be attained. With a given difference of pothe grid would be at the same potential as the plate if C were infinitely large; hence, it is desirable that C be as small as possible in order that there may be a large potential drop from the plate to the grid across C, as

Conversely, as the capacity value of is in-, creased, the potential on the plate with respect to the filament is decreased due to current flowing through O Hence, it is preferable that C be as small as possible, and that C be large as compared with C within the limits determined by the tuning ranges desired, by the voltage characteristic of the audion, by the output and input impedances of the audions employed, and by the practicable values of'capacities C" and C The reason for the desirability of this ratio of capacity values may also be described in another way, (see also 1 ig. 3), namely: that an input voltage impressed across the grid and filament, or input circuit '(A) does not introduce a voltage in the output circuit (B) because those two circuits are conjugate arms of a balanced eq'uivalent to introducing a conductance bridge, but this inputvoltage does introduce a voltage between the plate and filament across one arm, 0 of the bridge. This effect is across the grid circuit fromgrid to filament which increases with the ratio Q, making 1 the grid circuit less eflicient and consequently decreasing the degree of amplification.

Hence, the ratio should be as small as possible. v For the reasons above pointed out, the

value of the capacity 0 added between the plate and filament should be large compared with the natural inherent plate-grid capacity, C. Hence, since the magnitude of the inherent plate-filament capacity is normally comparable to the magnitude of the gridplate capacity, the value of the total capacity .at 0, between the plate and filament elec trodes will likewise be substantially larger than that of the natural inherent plate-lilament capacity.

I claim:' 1. An amplifier including an audion having an input circuit and an output circuit, one

termmal of said output circuit being connected to the plate electrode of said audion and through a capacity to the filament circuit of said audion, the secon'd terminal of said output circuit being connected through a second capacity to the grid electrode of said audion and through a third capacity-to said filament circuit, whereby undesirable capacitive coupling between said grid and plate elec trodes is neutralized.

2. An amplifier including an audion and a condenser-tuned inductance, said audion having undesirable capacitive coupling between the grid and plate electrodes thereof, and means for neutralizing said capacitive coupling comprising a capacity effectively connected between the plate electrode and the filament, and a second and third capacity efteetivel connected in series between the grid electro e and the filament, said inductance being connected from the plate electrode to a point between said second and said thirdnientioned capacities.

3. In an amplifier, an audion having undesirable capacitive coupling between the plate and grid electrodes thereof, an inherent capacity between the plate and the filament, an output circuit for said audion including a coil, and means for neutralizing said undesirable coupling comprising a capacity between the plate and the filament, said capacity being at least in part said inherent capacity, and two condensers connected in series between the grid and the filament, said coil being connected from the plate to a point between said two condensers.

4. A radio-frequency amplifier comprising two audions, each audion including grid, plate and filament electrodes, the filament electrodes ot-said audions being connected together. a coupling system interposed between said audions, said coupling system including a single radio-frequency coil associated with a blocking condenser and being connected between the plate electrode of'one audion and the grid electrode of the other audion and including four capacities effectively connected two from the terminals of said coil to the filaments and two,from the terminals of said coil to the remaining electrode of one of said audions.

1 5. A radio-frequency amplifier comprising two audions whose filaments are connected together through a filament system, each audion including grid and plate electrodes, a coupling system interposed between said audions, said coupling system including a single radiofrequency coil associated with a blocking condenser and being connected between the plate electrode of one audion and the grid electrode of the other audion and including four capaciticseffectively connected two from the terminals of said coil to the grid electrode of the first audion and two from the terminals of said coil to the filament system.

6. A radio-frequency amplifier comprising two audions whose filaments are connected together through a filament system, each audion including grid and plate electrodes, a coupling system interposed between said audions, said coupling system including a single radioirequcncy coil associated with a blocking condenser and being connected between the plate electrode of one audion and the grid electrode of the other audion and including four capacities effectively connected two from the terminals of said coil to the grid electrode of the first audion and two from the terminals of said coil to the filament system, and a fifth capacity onnected in parallel with said coil for tuning the same to radio frequencies.

7. A radio-frequency amplifier comprising two audions whos. filaments are connected together through a filament system, each audion including grid and plate electrodes between which exists an inherent coupling capacity, an interposed coupling system including a single radio-frequency coil associated with a blocking condenser and connected between the plate electrode of one audion and the grid electrode of the other audion and including four capacities, one of which comprises said grid-plate capacity, effectively connected two from the terminals of said coil to the grid electrode of the first audion and two from the terminals of said coil to the filament. system, said four capacities together comprising a balanced bridge by means of which undesirable reactions due to the coupling introduced by said grid-plate capacity are substantially eliminated.

8. A radio-frequency amplifier comprising two audions, each audion including grid, plate and filament electrodes. the filament electrodes of said audions being connected together, inherent coupling capacity between said grid and plate electrodes, a coupling system interposed between said audions, said coupling system including a single radio-frequency coil associated with a blocking condenser and connected between the plate electrode of one audion and the grid electrode of the other audion and including fourcapacities, one of which includes said grid-plate capacity, effectively connected two from the terminals of said coil to the filaments and two from the terminals of said coil to the remaining electrode of one of said audions, said four capacities together comprising a balanced bridge by means of which undesirable reactions due to the coupling introduced by said grid-plate capacity are substantially eliminated. I

9. A radio-frequeney amplifier comprising two audions, each audion including grid, plate and filament electrodes. the filament electrodes of said audions being connected together, a coupling system interposed between said audions, said coupling system including a ing two audions, each audion including grid, plate and filament electrodes, the filament electrodes of said audions being connected together, inherent coupling capacity between said grid and plate electrodes, a coupling system interposed between said audions, said coupling system including a single radio-frequency coil associated with a blocking condenser and connected between the plate electrode of one audion and the grid electrode of the other audion and including four capacities, one of which includes said grid-plate capacity, effectively connected two from the terminals of said coil to the filaments and two from the terminals of said coil to the remaining electrode of one of said audions, said four capacities together comprising a balanced bridge by means of which undesirable reactions due to the coupling introduced by said grid-plate capacity are substantially eliminated, and a fifthcapacity connected in parallel with saidcoil for tuning the same to radio frequencies.

11. A radio-frequency amplifier comprising two audions whose filaments are con nected together through a filament system, each audion including grid and plate electrodes between which exists inherent coupling-capacity, an interposed coupling system including a single radio-frequency coil associated with a blocking condenser and connected between the plate electrode of one audion and the grid electrode of the other audion and including four capacities, one of which comprises said grid-plate capacity, effectively connected two from the terminals of said .coil to the grid electrode of the first audion, and two from the terminals of said coil to the filament system, said four capacities together comprising a balanced bridge by means of which undesirable reactions due to the coupling introduced by said grid-plate capacity are substantially eliminated, and a fifth capacity connected in parallel with said coil for tuning the same to radio-frequencies.

12. In an audion amplifier having an input circuit and an output circuit connected to elements of an audion, and a natural capacity inherent between two of said elements whereby undesired reactions are produced, the method of reducing substantially to zero said undesired reactions which consists in introducing in said output circuit a plurality of capacitive reactions without introducing inductive reactions, one of said capacitive reactions being due to an inter-element capacity, dividing said capacitive reactions into two portions and balancing one of said portions against the other of said portions.

13. In an amplifier including an audion having grid. plate and filament electrodes, an input circuit connected to said grid, an outputcircuit connected to said plate, and natural inherentcapacity efi'ectively connected between said grid and plate electrodes,

: whereby undesired reactions are produced,

and balancing one of said portions against the other of said portions 14. The combination with an audionhaving grid, plate and filament electrodes of a circuit connected to the plate and containing two condensers, a connection from the filament to a point between the said condensers,

I a circuit connected to the grid, and a capacity inserted between the two circuits to balance the capacity between the grid and plate of the audion. 15. The combination with an ,audionhaving grid, plate and filament electrodes and having an input circuit and an output cir-- cuit, of a capacity in the output circuit divided into two parts and having a lead connected from its midpoint to the filament and an end connected to the plate electrode, and

a balancingcapacity connected between the grid electrode and the other end of the divided capacity.

16. An audion amplifier stage having an input circuit, an output circuit, an audion capacitively coupling said circuits, said input circuit being connected between the grid and filament electrodes of said audion, and a balanced network of pure capacities only, including the couplingcapacity, and having as conjugate arms said input and output circuits.

17. An electric system including an input circuit, an output circuit, an audion capacitively coupling said circuits, and a balanced network'of pure capacities only, including the coupling capacity as one arm of said network and a capacity between the plate and filament electrodes of said audion as another arm of said network, said-input and output clrcuits being connected respectively as conugate arms of said network.

18. An electric system including an input circuit, an output circuit, an audion capacitively coupling said circuits, a balanced net-' work of pure capacities only,- including the coupling capacity as one arm of said network and a capacity between the plate and filament electrodes of said audion as another arm of said network, said input and output circuits being connected respectively as conjugate arms of said network, and means for supply- 1 ing direct current to said plate electrode.

19. A multi-stage amplifying system in-' cluding an input circuit, an output circuit and an audion in each stage, the gride-plate inherent capacity of each audion coupling the input and output circuits of each stage, and a balanced network of pure capacities only, including said. grid-plate coupling capacity asone arm of said network and a capacity between the plate and filament electrodes of said audion as another arm of said netwcrk, said input and output circuits of each stage beingconnected respectively as conjugate arms of said network.

20. A multi-stage amplifying system including an input circuit, an output circuit and an audion in each stage, the grid-plate inherent capacity of each audion serving to couple the input and output circuits of each stage, a balanced network of pure capacities only, including said grid-plate capacity as one arm of said network and a capacity between the plate and filament electrodes of said audion as another arm of said network, said input and output circuits of each-stage being connected respectively as conjugate arms of said network, and means for supplying direct current to the said plate electrode of the audion in each stage,

v 21. A multi-stage amplifying system having in each stage an input circuit, a tuned output circuit,-an audion capacitively coupling said circuits, abalanced networkpf pure capacities only, including the coupling capacity as one arm of said network and a capacity between the plate and filament electrodes of said audion as another arm of said network, said input and; output circuits being connected respectively as conjugate arms of said network.

22. An amplifying system having an input circuit, an output circuit, an audion capacitively coupling said circuits, the output circuit of said audion being linked to the input circuit of a succeeding audion by means including a tunable circuit, and a balanced network of pure capacities only, including the coupling capacity as one arm of said network and a capacity between the plate and filament electrodes of said first mentioned audion as another arm 'ofsaid network, said input and output circuits being connected respectively as conjugate arms of said network, I

23. An audion amplifier stage having an input circuit, an output circuit, an audion capacitively coupling said circuits, the plate circuit of said audion being linked to the grid circuit of a succeeding audion by capacitive coupling means, and a balanced network of pure capacities only, including said coupling capacity as one arm of said network and a capacity between the plate and filament of said first-mentioned audion as another arm of said network, said input and output circuits being connected respectively as conjugate arms of said network.

24. An electric system including an input circuit, an output circuit, an audion having grid-plate capacity coupling a said circuits.

and a balanced network of pure capacities only, including said grid-plate capacity as oneiarm of said network and a substantially larger capacity between the plate and lilament electrodes of said audion as another arm of said network, said input and output circuits being connected respectively as conjugate arms of said lie-twink.

25. An electric system including an input circuit, an output circuit, an audion having grid plate capacity coupling said circuits,

and a balanced network of pure capacities only, includingsaid grid-plate capacity as one arm of saidaietwork, and an added capacity between the plate and filament electrodes of said audion, said added capacity being substantially larger than the inherent only, including said grid-plate capacity as one arm of said network and a capacity between the plate and filament electrodes of said audion as another arm oi said network, said input and output circuits being connected, respectively, as conjugate arms of said network,

the ratios of the values of the capacities of said network being such that the alternating current voltagerintroduced between said plate and filament electrodes is minimized.

27. An electric system includingan input circuit, an output circuit, an audion having grid-plate capacity coupling said :i rcuits, an d.

a balanced network of pure capacities only. in-

cluding said grid-plate capacity as one arm of said network and a capacity of the order of one hundred times larger than said gridplate capacity between the plate and filament electrodes of said audion as another arm of said network, said input and output circuits being connected, respectively, as conjugate arms of said network.

28. An amplifier including an audion having an input circuit and an output circuit, one terminal of said output circuit being connected to the plate electrode of said audion, and thru a capacity to the filament circuit of said audion, the second terminal of said output circuit being connected thru a second capacity to the grid electrode of said audion, and thru a, third capacity to said filamentcircuit, said first-named capacity being the largest and said second-namedcapacity being the smallest, whereby undesirable capacitive coupling between said grid and plate electrodes is neutralized and the amplification is maintained at a high degree.

29. An amplifier-including an audion having an input circuit and an output circuit, one

terminal of said output circuit being connected to the plate electrode of said audion and thru a capacity to the filament circuit of said audion, the second terminal oi said outputcircuit being connected thru a second 'apacity to the grid electrode of said audion, and thru a third capacity of a value substantially less than, that of said first-named capacity to said filament circuit, whereby undesired capacitive coupling between grid and plate electrodes is neutralized and the amplification is maintained at a high degree.

30. In a radio-frequency amplifier, two audions, each'of which has grid, plate'and filament electrodes, the filament electrodes of said audions being connected together thru a filament system, and a coupling system interposed between said audions, said coupling system including a radio-frequency coil and tour capacities effectively connected two from the terminals of said coil to the grid electrode of the first audion and 'two from the terminals of said coil to said filament system, one of said last-named two capacities being substantially larger than the other whereby a step-up voltage ratio is maintained in said coupling system.

31. A radio-frequency amplifier including two audions whose filaments are connected together thru a filament system. each audion having grid and plate electrodes. :1 couplin system interposed between said audions. said coupling system including a radio-frequency coil and essentially four capacities, one of the terminals of said coil being connected thru the first of said capacities to the grid electrode of the first of said audions and thru the second of said capacities to the filaments of said audions. the other terminal oi said coil being connected thru the third of said capacities to the grid electrode of the first said andion. and thru the fourth of said capacities to the filaments ofsaid audions. the input terminals of said coupling system being e1 fe'ctively the terminals of said second capacity and the output terminals of said coupling system being effectively the terminals of said fourth capacity. the value of said second capacity being substantially greater than the value of said fourth capacity. whereby a stepup ratio is maintained in said coupling system;

32; In an amplifier stage. including an in put circuit' and an output circuit, an audion having grid. plate and filament electrodes and grid-plate capacity coupling said circuits, and an impedance bridge balanced substantially throughout a desired frequency range and of which said input and output circuits are eonjugatearms, said bridge including as one impedance arm the gridsplate capacity of said audion, and as another arm an impedance between the plate and filament of saidaudion. the impedance of said grid-plate capacity being substantially greater than said impedance between plate and filament throughout a high degree.

character throughout said desired frequency Patent No. 1,757,494.

range, one-of said remaining three arms being connected between said plate and filament electrodes and having an impedance substantially smaller than that of said grid-plate coupling capacity, whereby the amplification of. said stage is maintained at a high degree. In testimony whereof I aflix my signature.

HAROLD A. WHEELER.

Certificate of Correction;

'fGranted'May 5,1930, to

It is hereby certified 1 that error appear-sin "theiprinted "specifihation of the above numbered'patent requiring correction as followsIaPage-3,l nes 78-and 79,;

for the ratio read page :6, -claiin-ii for 'the misspelled corri- 2 2: i e pound word ,gride-plate read g -j2late; and that the said Letters .Patent should be read with these corrections thereinTtliat-athe same may conform to the record of the case in the Patentflflice;

, Signed and ,sealed this am;- day of May, A41); 1930;

a .iiLJQMQOREf -f 

